add a bake migration task

I started to create a bake task that can generate migration files with tables from a database.

This can be interesting to run this command when you already created database tables (with phpMyAdmin or whatever the tool) and you want to export your tables in a phinx file for the migrations plugin.

For now, it's possible to:
- create a migration file in a plugin
- you can change the default connection
- create the tables only if models exist or force creation for all tables

If someone wants to improve the code or has other ideas to make this better, I'm taking advices. :)

Other functionnalities I want to add are: 
- improve columns defintion for generated tables
- add only the changes if there are already other previous migrations files
- add data from the database in phinx migration file
You could use Inflector::underscore(). That would also make `CamelCaseName` valid input as a bonus :)